Obesity continues to be among the leading preventable causes of morbidity and mortality in the United States and much of the rise in obesity is attributable to food choice. It is well-established that food choices (quality, quantity are affected by social and physical environments. Recent research has suggested that obesity might be transmitted through social networks. Strategies to reduce obesity risk could be designed to take advantage of the relationship between social connections and obesity, but further investigation of the interactions between social and physical environments and food choice is needed to help identify possible mechanisms that could be leveraged to improve health. The workplace is an ideal setting to study the relationship between social connections and food choice. Employed individuals spend roughly half their waking hours in the workplace and nearly everyone eats during the workday. Workplace eating establishments (e.g. cafeterias) provide a unique opportunity to study obesity prevention strategies that capitalize on the interactions of established social connections and the food purchasing environment. Since 2010, we have undertaken a series of interventions within the Massachusetts General Hospital (MGH) food services system and studied their impact on purchases of healthy food items. All food items sold at MGH have been assigned traffic light labels informed by USDA dietary guidelines with clearly communicated messaging to indicate the healthfulness of food choices. We have found that providing employees with social norms feedback (peer comparisons) increases healthy food choices. In preliminary analyses, we have also identified associations in healthy/unhealthy food purchasing behaviors among employees whose social connections have been inferred from cafeteria and administrative data. In the current proposal, we use the MGH cafeteria system to test whether the relationship between social connectedness and food choice varies by employee characteristics and specific food environments (Aim 1). Using data from a prior randomized trial testing the influence of social norms feedback on healthy food choices, we will determine whether the positive effects of the intervention on employees' food choices are amplified by their social connections and transmitted through social connections to their co-workers (Aim 2). Our established, objective data collection system in this real-world food choice laboratory allows us to conduct novel investigations of worksite social networks and the interaction of these factors with the food environment. Results from this research will position us to develop and test new interventions that can leverage social connectedness and the food environment to improve food choices and reduce obesity risk.